Conventional microwave amplifiers include an internally matched microwave amplifier. The internally matched microwave amplifier includes a package within which housed are a high-power field effect transistor for amplifying microwave signals, an input-side matching circuit connected to the input of the field effect transistor, and an output-side matching circuit connected to the output of the field effect transistor. The input-side matching circuit is for matching the input impedance of the field effect transistor with the impedance of a signal source external to the package, and the output-side matching circuit is for matching the output impedance of the field effect transistor with the impedance of a load external to the package.
As shown in an equivalent circuit in FIG. 1, such a microwave amplifier includes a high power field effect transistor 10 for microwave signals. Transistor 10 has its source grounded, has its drain connected through an inductor 12 to an output-side matching circuit 14, and has its gate connected through an inductor 16 to an input-side matching circuit 18.
FIG. 2 schematically shows an arrangement of the microwave amplifier of FIG. 1 within a package. In FIG. 2, output-side matching circuit 14 and input-side matching circuit 18 are separate from field effect transistor 10 and are disposed on substrates 20 and 22, respectively, with the drain of transistor 10 connected to output-side matching circuit 14 by means of a plurality of gold wires 12a which serve as inductor 12, with the gate of transistor 10 connected to input-side matching circuit 18 by means of a plurality of gold wires 16a which serve as inductor 16.
Field effect transistor 10 has a large gate width in order to provide a high power output, i.e. large drain current, and, accordingly, its gate-source capacitance is large and its gate-source resistance value is small. Thus, the input impedance, i.e., the impedance exhibited between the gate and the source, is capacitive, and has a small value, e.g. 0.2-0.3 .OMEGA.. It is, therefore, necessary to match this input impedance with the impedance of, for example, 50 .OMEGA. of a circuit to be connected to the input-side of the microwave amplifier. For that purpose, the number of gold wires 16a is adjusted to adjust the inductance value of inductor 16 between input-side matching circuit 18 and the gate of field effect transistor 10 so as to cancel the capacitive component of the input impedance as much as possible, before the input impedance of the amplifier circuit is matched with the impedance of the circuit connected to the input-side by input-side matching circuit 18.
When such a microwave amplifier is used in a wide band, e.g. within a range of from 3 GHz to 12 GHz, the impedance of inductor 16 provided by gold wires 16a will vary with the frequency of an input signal and the impedance observed from the junction between input-side matching circuit 18 and inductor 16 toward field effect transistor 10 varies with the input signal frequency. Thus, it is impossible for the microwave amplifier to have a flat frequency response characteristic and a high gain.